fix(common): let case conversion pipes accept type unions with null (#36259) (#37447)

The old implementation of case conversion types can handle several
values which are not strings, but the signature did not reflect this.

The new one reports errors when falsy non-string inputs are given to
the pipe (such as `false` or `0`) and has a new signature which
instead reflects the behaviour on `null` and `undefined`.

Fixes #36259

BREAKING CHANGE:
The case conversion pipes no longer let falsy values through. They now
map both `null` and `undefined` to `null` and raise an exception on
invalid input (`0`, `false`, `NaN`) just like most "common pipes". If
your code required falsy values to pass through, you need to handle them
explicitly.

PR Close #37447

goldens/public-api/common/common.d.ts

@@ -194,6 +194,8 @@ export declare abstract class LocationStrategy {
 
 export declare class LowerCasePipe implements PipeTransform {
     transform(value: string): string;
+    transform(value: null | undefined): null;
+    transform(value: string | null | undefined): string | null;
 }
 
 export declare class NgClass implements DoCheck {
@@ -392,6 +394,8 @@ export declare type Time = {
 
 export declare class TitleCasePipe implements PipeTransform {
     transform(value: string): string;
+    transform(value: null | undefined): null;
+    transform(value: string | null | undefined): string | null;
 }
 
 export declare enum TranslationWidth {
@@ -403,6 +407,8 @@ export declare enum TranslationWidth {
 
 export declare class UpperCasePipe implements PipeTransform {
     transform(value: string): string;
+    transform(value: null | undefined): null;
+    transform(value: string | null | undefined): string | null;
 }
 
 export declare const VERSION: Version;